Ion implantation has become a standard technique for introducing impurities into semiconductor wafers. The impurities determine the conductivity of the region into which they are implanted. Impurities are introduced into the bulk of semiconductor wafers by using the momentum of energetic ions as a means of imbedding them into the crystalline lattice of the semiconductor material. The fabrication process for integrated circuit devices usually includes several ion implantation steps for impurity doping of different device regions.
Ion implantation systems typically include an ion source for converting a gas or a solid material into a well-defined ion beam. The beam is mass analyzed to eliminate undesired ion species, is accelerated to the desired energy and is focused onto a target plane. The beam is deflected over the target area by beam scanning, by target movement, or a combination of scanning and target movement. One form of beam scanning utilizes two-dimensional electrostatic scanning over the target area utilizing a raster scan (see, for example, U.S. Pat. No. 4,283,631).
In the operation of ion implantation systems, it is necessary to measure the cumulative ion dose implanted in the semiconductor wafer, since the number of ions implanted determines the conductivity of the implanted region. Typically, ion implants are specified in terms of ion species, ion energy and dosage in ions per square centimeter. Continuous measurement of ion dosage is necessary, since ion sources do not deliver accurate, constant ion beam currents. It is further necessary to monitor the spatial uniformity of the implanted dose over the surface area of the wafer. Spatial uniformity variations outside specified limits will result in operating characteristics which vary from device to device. Semiconductor fabrication processes typically require dose accuracy within one percent and dose uniformity of less than one percent.
In the past, cumulative ion dose has been measured by a Faraday cup positioned in front of the target wafer. The ion beam passes through the Faraday cup to the wafer and produces a current in the Faraday. The wafer itself is part of the Faraday system and cannot be grounded. The Faraday current is supplied to an electronic dose processor which integrates the current with respect to time to determine the total ion dosage. Dose uniformity has been monitored by a corner cup arrangement. A mask having a central opening is positioned in the path of the ion beam. The beam is scanned over the area of the mask with the portion passing through the central opening impinging on the target wafer. Small Faraday cups are located at the four corners of the mask and sense the beam current at these locations. Individual conductors connect the four corner cups to a monitoring system which determines the deviation of the beam current at each corner from an average value. In some systems, the corner cups have been connected in common for measurement of cumulative ion dose.
It is a general object of the present invention to provide improved dose measurement and uniformity monitoring apparatus for use in a scanned charged particle beam system.
It is another object of the present invention to provide dose measurement and uniformity monitoring apparatus which is simple in construction and economical to manufacture.
It is a further object of the present invention to provide dose measurement and uniformity monitoring apparatus for a scanned charged particle beam system wherein a beam sensor having a single output current lead is used to determine ion dose and ion dose uniformity simultaneously.